1. Field of Invention
The present invention relates to an electrical circuit comprising bus bars, and to an electrical junction box having the circuit. The circuit and the junction box are applicable particularly in a vehicle.
2. Description of Related Art
Recently, rapid increase of the number of electrical and electronic component parts mounted on a vehicle has led to increase of the number of circuits to be accommodated in an electrical connection box and in particular a junction box for a vehicle. Thus, in forming branch circuits at high density, it is necessary to mount a large number of component parts in the junction box, which causes increase in number of manufacturing stages.
In a typical known junction box shown in FIG. 7, bus bars 5A-5D are overlaid one upon another between an upper case part 2 and a lower case part 3, with insulation plates 4A-4E interposed between them. The upper case 2 has connector sockets 2a, relay sockets 2b, and fuse sockets 2c on which in use connectors 6, relays 7, and fuses 8 are respectively mounted. Terminals of the connectors 6, the relays 7, and the fuses 8 are connected to terminals 5a projecting from the bus bars directly or through relaying terminals. The lower case part 3 also has connector sockets 3a to connect connectors to terminals projecting from the bus bars.
In the junction box 1, with the increase in the number of circuits, the area and the number of layers of the bus bars increase, and thus the junction box becomes large. In the case where the connector sockets, the relay sockets and the fuse sockets are mounted on both the upper and lower case parts, it is possible to make the area of the junction box smaller than in the case where these sockets are mounted on only the upper case part or the lower case part.
However, in the case where the sockets are mounted at vertically opposed positions of the upper and lower case parts, terminals bent from the bus bars abut or overlap on each other and thus cannot be moved or distributed. In this case, it is necessary to form terminals on bus bars of additional layers. The increase of number of the layers of the bus bars leads to the increase of the height of the junction box. That is, the junction box is necessarily large.
Further, the connectors, fuses and relays are connected to the bus bars disposed inside the junction box. Thus, when the specification of the connection between the internal circuit and the fuses as well as the relays is altered, it is necessary to alter the construction of the entire internal circuit. That is, the above-described junction box is incapable of easily permitting a circuit alteration.
It is possible to deal with the problem of the increase of the layers of the bus bars caused by the overlapping or abutting of the terminals formed on the bus bars, by separate disposition of the bus bars to be connected to the connectors, those to be connected to the fuses and those to be connected to the relays. Further, it is possible to make the junction box more easily adjustable to various kinds and grades of vehicles by separating the internal circuit of the junction box into a connector connection circuit, a relay connection circuit, and a fuse connection circuit serving as a connector module, a relay module, and a fuse module.
The overall required internal circuit can be constructed of bus bars of the connector module, the relay module, and the fuse module by joining them to each other. The bus bars can be joined to each other by welding together projections at the ends of bus bars by resistance welding or the like.
As the material for a bus bar circuit, mostly, tin-plated brass sheet having a thickness of 0.64 mm is used in consideration of favorable weldability and low cost. In connecting the bus bars to each other by resistance welding, it is possible to form a projection on a welding surface of one bus bar so that the two bus bars are welded to each other by projection welding which is a form of resistance welding. Thus it is conceivable to use tin-plated brass as the material for the bus bar of each module and connect the modules to each other by projection welding.
However, depending on the required specification of the junction box, a current value higher than a normal value (about 20 A) or a much higher current value (about 100 A) may be applied to the internal circuit of the junction box. In the case where the internal circuit to which high current should be applied is composed of bus bars made of a material having a low conductivity, the bus bars generate heat owing to the internal resistance of their material, when high current is applied. Consequently, the temperature inside the junction box rises, which leads to disadvantages. Brass, which is conventionally used as the material of the bus bar, has a conductivity less than 30%. Therefore, brass is unsuitable for a bus bar circuit to which an electric current having a value higher than the normal value should be applied. That is to say, a bus bar circuit made of brass is not reliable in its operation.
The generation of heat can be suppressed if a high-conductivity material is used for the bus bar. However, such a high-conductivity material generates little heat when power is applied during welding. Thus it is difficult to heat the high-conductivity material to a temperature necessary for melting when welding, so that sufficient welding strength is difficult to obtain. Therefore it is inappropriate to use such high-conductivity material for the bus bars when they are to be welded to each other.
It is an object of the present invention to provide a circuit, e.g. for an electrical junction box, containing bus bars in which the construction is particularly adapted to the current levels to be applied to the bus bars.
It is a further object to provide an electrical junction box containing bus bars, which can be of compact dimensions.
According to the present invention, there is provided an electrical circuit having two bus bars welded to each other at a weld, wherein a first one of the two bus bars is formed of a material having an electrical conductivity selected from (a) in the range of from about 10 to about 60% and (b) not less than about 60% and the second one of the two bus bars is formed of a material having an electrical conductivity of not less than about 60%, and wherein at least one of the bus bars is tin-plated and at the weld the bus bars are joined through a tin-plating layer.
In this specification, including the claims, electrical conductivity is expressed in %, according to IACS (IACS is the abbreviation for International Annealed Copper Standard, which relates the electrical conductivity of a metal or alloy to that of copper in percentage terms).
As described above, the bus bar circuit of the present invention in this aspect may be formed of a novel combination of materials welded to each other to allow the bus bar circuit to have performance complying with a specified current value required for the junction box. For example, in the case where a specified value of electric current to be applied to the junction box is higher than a normal value (e.g. about 20 A) but not higher than a high value (e.g. about 100 A), it is possible to form one bus bar of a material having a low conductivity and the other bus bar of a material having a high conductivity. In this case, it is possible to achieve a high conductivity of a part of the bus bar circuit and thus partly suppress the heat output in use due to electrical resistance, which allows reliability of the bus bar circuit to be secured. Further, because the low-conductivity material for one bus bar is inexpensive, it is possible to produce the bus bar circuit at a low cost by using the combination of the low-conductivity material and the high-conductivity material.
In the case where a specified value of electric current to be applied to the junction box is high, both bus bars welded together can be formed of high-conductivity materials. As described previously, it is difficult to secure a desired high degree of welding strength when high-conductivity material is used for one bus bar or both bus bars. But according to the present invention in this aspect, because at least one of the two bus bars is tin-plated, the electric resistance at the weld increases owing to the melting of the plated layer when electric current is passed to weld them to each other. It is believed that the melted tin may alloy with the materials of the bus bars at the weld, and that the alloyed portions adhere well to each other. Good weldability can be secured, and a desired degree of welding strength can be obtained. Tin-plating of both bus bars causes the electric resistance to increase more, which further improves the weldability.
In the case, for example, where a specified value of electric current to be applied to the junction box is not less than about 20 A nor more than about 100 A, as a preferable combination of bus bar materials it is possible to use tin-plated brass as the material for one bus bar and any one of a tin-plated copper alloy, copper alloy not tin-plated, tin-plated oxygen-free copper, and oxygen-free copper not tin-plated as the material for the other bus bar. As another suitable combination, it is possible to form one bus bar of brass not tin-plated and the other bus bar of the tin-plated copper alloy or the tin-plated oxygen-free copper.
Brass is a low-conductivity material having a conductivity lower than 30%. On the other hand, copper alloy is a high-conductivity material having a conductivity of typically about 60%. Oxygen-free copper is also a high-conductivity material having a conductivity of typically about 100%. Within the present invention, as described above, one bus bar may be formed of the low-conductivity brass and the other bus bar may be formed of the high-conductivity copper alloy or the oxygen-free copper, and at least one of the bus bars may be tin-plated to secure a favorable degree of weldability. In this manner, it is possible to form a bus bar circuit which satisfies the specification for the junction box and is inexpensive.
In the case where a specified value of electric current to be applied to the junction box is high, as a preferable combination of bus bar materials, one bus bar may be formed of a tin-plated copper alloy or tin-plated oxygen-free copper, and the other bus bar may be formed of any one of a tin-plated copper alloy, a copper alloy not tin-plated, tin-plated oxygen-free copper, and oxygen-free copper not tin-plated. Even though the high-conductivity materials are used for both bus bars, at least one of both bus bars is tin-plated. Thus, it is possible to obtain a desired degree of weldability and satisfy the specification for the junction box. It is possible to improve weldability even further when both bus bars are tin-plated, at least at the welding zone.
According to the present invention in a second aspect, there is provided an electrical circuit having two bus bars welded to each other at a weld, wherein a first one of the two bus bars is formed of brass without tin-plating and the second one of the two bus bars is formed of a material selected from tin-plated brass and brass without tin-plating.
In the case where a specified value of electric current to be applied to the junction box is a normal current value (e.g. about 20 A), brass having a low conductivity can be used as the material for both bus bars to be welded to each other. Because the brass has a high internal resistance, it is heated to a temperature necessary for melting it when current is applied in welding. Thus, it is possible to secure a desired degree of welding strength without tin-plating one bus bar or both bus bars. Accordingly, cost is reduced since the manufacturing cost of bus bars made of the brass not tin-plated is less than that of bus bars made of tin-plated brass.
The bus bars are preferably welded to each other by resistance welding. Owing to the use of the resistance welding, it is possible to improve production efficiency in the welding process and stabilize welding quality. Alternatively, ultrasonic welding, gas welding and/or laser welding may be used.
A projection may be formed on a welding surface of one of the two bus bars to be welded to each other by resistance welding. The projection allows a welding force and electric current to concentrate at the contact portion of the projection, thus increasing the weldability. In addition, the projection enhances the welding strength in synergism with tin-plating. In the case where the projection is formed on one bus bar having a higher hardness than the other bus bar, they can be welded to each other. To obtain a high degree of welding strength, it is favorable to form the projection on the other bus bar having a lower hardness, because the projection can be smoothed away in the projection welding.
According to the invention in a further aspect, the above circuits of the invention, in particular the materials selected for the welded bus bars, are applied to an electrical junction box for a vehicle having a casing and mounted within the casing a first circuit module adapted to be connected to at least one electrical connector in use of said electrical junction box and a second circuit module selected from
(i) a fuse module adapted to be connected to electrical fuses in use of the electrical junction box,
(ii) a relay module adapted to be connected to electrical relays in use of the electrical junction box, and
(iii) a combined fuse and relay module adapted to be connected to electrical fuses and electrical relays in use of the electrical junction box,
each of the first and second circuit modules having at least one bus bar and an insulating body carrying the at least one bus bar,
two of the bus bars, respectively on the first and second circuit modules, being welded to each other at a weld to form electrical connection between the first and second circuit modules.
Conventionally, in an electrical junction box for a vehicle, one conductive plate is punched to form the connector circuit, the fuse circuit, and the relay circuit, and connector tabs, fuse tabs, and relay tabs are provided on the connector circuit, the fuse circuit, and the relay circuit respectively. Thus, disposition and handling of the circuits are complicated. Consequently, the bus bars occupy a wide area and are multi-layered.
On the other hand, with the electrical junction box of the present invention, the connector connection bus bar, the fuse connection bus bar, and the relay connection bus bar may be formed by punching separate conductive plates and are disposed at separate positions. Therefore, it is possible to reduce layering of the bus bars and it is easy to dispose and handle the circuits. Thus it is possible to reduce the area and/or volume of the bus bars. In joining the bus bars of each module by welding them to each other, the bus bars made of the above-described materials are combined with each other to form various bus bar circuits, according to specified values of electric current to be applied to the junction box.
The invention further includes a vehicle having the electrical junction box mounted in it.